Here's an interesting conundrum posed by Przemyslaw Pokrywka. One
of the refactorings in the book
is Introduce Null Object - a very
useful refactoring (also discussed in Josh's new
book.) Przemyslaw's point is that this refactoring can alter
behavior. If you have a method return a null, and you invoke a
method on that null you'll get a null pointer exception. If you use a
Null Object you'll get some default behavior.

Now many refactorings do alter behavior, indeed they are intended
to. If you apply Form Template Method, then the program works
differently. The key question is whether this is what in my
DefinitionOfRefactoring I called observable
behavior. That is does it change what the program essentially
does? With Introduce Null Object you have to look around the
program for places that manipulate returned references,
typically by checking to see if it's null. This is what makes this
a rather tricky refactoring.

The interesting part of the conundrum is what happens if you miss
an area where there's a bug. Somewhere in your program you invoke a
method on a null reference. Before the refactoring you'd get an
exception, we'll assume here it's one that you don't know about and
goes all the way up to some top level handler. After the refactoring
you get the default behavior - which may in fact fix the bug. If you
fix a bug you don't know about is this still a refactoring?

I'd argue yes, since you didn't know or care (enough) about the
buggy behavior then I'd say that behavior wasn't
observable. Even if you knew about the bug, I'd still argue that
it's okay to call this a refactoring if the bug wasn't behavior
you cared about preserving.

This is an interesting case, and I could easily imagine myself
changing my mind or exploring more edge cases.

One of the interesting things it points out is the difference
between manual and tool driven refactoring. With manual refactoring
you can make judgement calls like this, tools often have to be much
more careful. Yet even tools can't always guarantee to preserve
behavior - even a rename method can be break on refactoring if that
method is called by reflection with a name that's read from a file.

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